This invention relates to a yarn feed mechanism for a tufting machine and more particularly to a scroll-type pattern controlled yarn feed wherein each set of yarn feed rolls is driven by an independently controlled servo motor. A computerized design system is also provided because of the complexities of working with the large numbers of individually controllable design parameters available to the new yarn feed mechanism.
Pattern control yarn feed mechanisms for multiple needle tufting machines are well known in the art and may be generally characterized as either roll-type or scroll-type pattern attachments. Roll type attachments are typified by J. L. Card, U.S. Pat. No. 2,966,866 which disclosed a bank of four pairs of yarn feed rolls, each of which is selectively driven at a high speed or a low speed by the pattern control mechanism. All of the yarn feed rolls extend transversely the entire width of the tufting machine and are journaled at both ends. There are many limitations on roll-type pattern devices. Perhaps the most significant limitations are: (1) as a practical matter, there is not room on a tufting machine for more than about eight pairs of yarn feed rolls; (2) the yarn feed rolls can be driven at only one of two, or possibly three speeds, when the usual construction utilizing clutches is used--a wider selection of speeds is possible when using direct servo motor control, but powerful motors and high gear rotors are required and the shear mass involved makes quick stitch by stitch adjustments difficult; and (3) the threading and unthreading of the respective yarn feed rolls is very time consuming as yarns must be fed between the yarn feed rolls and cannot simply be slipped over the end of the rolls, although the split roll configuration of Watkins, U.S. Pat. No. 4,864,946 addresses this last problem.
The pattern control yarn feed rolls referred to as scroll-type pattern attachments are disclosed in J. L. Card, U.S. Pat. No. 2,862,465, are shown projecting transversely to the row of needles, although subsequent designs have been developed with the yarn feed rolls parallel to the row of needles as in Hammel, U.S. Pat. No. 3,847,098. Typical of scroll type attachments is the use of a tube bank to guide yarns from the yarn feed rolls on which they are threaded to the appropriate needle. In this fashion yarn feed rolls need not extend transversely across the entire width of the tufting machine and it is physically possible to mount many more yarn feed rolls across the machine. Typically, scroll pattern attachments have between 36 and 120 sets of rolls, and by use of electrically operated clutches each set of rolls can select from two, or possibly three, different speeds for each stitch.
The use of yarn feed tubes introduces additional complexity and expense in the manufacture of the tufting machine; however, the greater problem is posed by the differing distances that yarns must travel through yarn feed tubes to their respective needles. Yarns passing through relatively longer tubes to relatively more distant needles suffer increased drag resistance and are not as responsive to changes in the yarn feed rates as yarns passing through relatively shorter tubes. Accordingly, in manufacturing tube banks, compromises have to be made between minimizing overall yarn drag by using the shortest tubes possible, and minimizing yarn feed differentials by utilizing the longest tube required for any single yarn for every yarn. The most significant limitation of scroll-type pattern attachments, however, is that each pair of yarn feed rolls is mounted on the same set of drive shafts so that for each stitch, yarns can only be driven at a speed corresponding to one of those shafts depending upon which electromagnetic clutch is activated. Accordingly, it has not proven possible to provide more than two, or possibly three, stitch heights for any given stitch of a needle bar.
As the use of servo motors to power yarn feed pattern devices has evolved, it has become well known that it is desirable to use many different stitch lengths in a single pattern. Prior to the use of servo motors, yarn feed pattern devices were powered by chains or other mechanical linkage with the main drive shaft and only two or three stitch heights, in predetermined ratios to the revolutions of the main drive shaft, could be utilized in an entire pattern. With the advent of servo motors, the drive shafts of yarn feed pattern devices could be driven at almost any selected speed for a particular stitch.
Thus a servo motor driven pattern device might run a high speed drive shaft to feed yarn at 0.9 inches per stitch if the needle bar does not shift, 1.0 inches if the needle bar shifts one gauge unit, and 1.1 inches if the needle bar shifts two gauge units. Other slight variations in yarn feed amounts are also desirable, for instance, when a yarn has been sewn at a low stitch height and it is next to sewn at a high stitch height, the yarn needs to be slightly overfed so that the high stitch will reach the full height of subsequent high stitches. Similarly, when a yarn has been sewn at a high stitch height and it is next to sewn at a low stitch height, the yarn needs to be slightly underfed so that the low stitch will be as low as the subsequent low stitches. In addition, some yarn feed rolls, particularly at the ends of the tufting machine, may experience relatively more yarn drag from the tube bank. Compensation for this additional drag can be provided by very slightly overfeeding the yarn on those rolls. Therefore, there is a need to provide a pattern control yarn feed device capable of producing scroll-type patterns and of feeding the yarns from each pair of yarn feed rolls at an individualized rate.